1. Field of the Invention
The invention relates to the field of power semiconductor electronics.
It proceeds from a thyristor in accordance with the preamble of the first claim.
2. Discussion of Background
Such a thyristor has already been described in European Patent Application EP 0 327 901 A1.
This is an asymmetrically blocking thyristor, that is to say a thyristor which has only a forward-blocking and a forward-conducting operating state. A reverse-blocking state is not required, but the necessary reverse-conducting state is achieved by means of a discrete or integrated antiparallel diode.
Because of the absence of reverse-blocking capability, such asymmetrically blocking components have some additional degrees of freedom in terms of design which make a different optimization possible. The most important possibilities for improvement are:
a) the introduction of a stop layer for the purpose of a thinner wafer design. PA1 b) the introduction of anode-side shorting for improving the dynamic characteristics. PA1 In the case of GTOs which do not have cathode-side shorting, the anode-side definition of a triggering threshold is also lost with the lightly doped barrier layer, i.e. especially at elevated temperatures, the components may become so readily-triggerable that the voltage-sustaining capability (dV/dt strength) is put at risk. PA1 In itself, a barrier layer over the entire surface intensifies the leakage currents in the component. PA1 The barrier layer results in a reverse-voltage behavior which is not actually defined since the p-n junction additionally introduced has a certain blocking capability and accordingly breaks down at a poorly defined voltage.
Both measures have a great potential for improvement, the stop layer primarily at very high voltages (for example, above 3 kV), the anode shorting primarily in the case of gate-turn-off high-power components, so-called GTOs, since the sweeping of charge carriers out of the anode-side regions is markedly facilitated in these components and the tail current is consequently reduced.
However, both measures have also certain disadvantages in relation to turn-on and on state since the effectiveness of the anode-side emitter is influenced. In the case of the stop layer, this influence is only very slight and gradual, but in the case of the shorting, it is much stronger since an actual triggering threshold is introduced. A combination of both measures in the same component has the strongest triggering-inhibiting action.
It is interesting that this opposing influencing of triggering and turn-off optimization manifests itself not only in relation to the presence of anode shorting but also in relation to the design of said shorting: the optimization of the turn-off behavior requires per se as fine-mesh shorting as possible in which the emitter and short regions are distributed as uniformly as possible over the entire active component surface, whereas the triggerability suffers to the extent that the minimum emitter width is reduced.
The design of the structure of a conventional shorted anode is accordingly an optimization problem in which it is a matter of having the best possible influence on the turn-off characteristics without, at the same time, impairing the readiness of the components to trigger in a way prohibited by specifications or a way which is no longer controllable by gate units.
The shorting action during turn-off is crucial especially at high injection in the n-type base, whereas the behavior of the anode emitter is important for the triggerability at low to medium injection. In the patent application EP 0 327 901 A1 mentioned, an attempt is therefore made to introduce other structures of lower dopant concentration in front of the actual emitter structure as active at high injection, which structures of lower dopant concentration markedly influence the characteristics at low injection. For this purpose, a lightly doped p-type barrier layer is introduced in front of the anode short circuits.
This has the consequence that the typical triggering threshold for the shorted anode, i.e. the defined anode current density at and above which hole emission can take place from the anode, becomes virtually ineffective. Consequently, the components are generally very much more readily triggerable. This measure, however, also has disadvantageous side effects: